Conventionally, as shown in FIG. 11, a flow controller (also referred to as “pressure-type flow controller”) including a main body block 3 having formed therein a flow passage 2 that connects between a gas inlet 2a and a gas outlet 2b, a restriction part OR interposed in the flow passage 2, a control valve 4 interposed in the flow passage 2 upstream from the restriction part OR, a first pressure detector 5a that detects the pressure in the flow passage 2 between the control valve 4 and the restriction part OR, and a controller 6 that controls the control valve 4 so as to achieve a predetermined flow rate based on a value detected by the first pressure detector 5a is known (Patent Document 1, etc.).
This control utilizes the principle that when a so-called critical expansion condition of (P1/P2) about 2 is maintained between an upstream pressure (P1) of the restriction part OR and a downstream pressure (P2) of the restriction part OR, a flow rate (Q) of the gas G flowing through the restriction part OR, such as an orifice, establishes the relation Q=KP1 (K is a constant).
Based on this principle, the control valve 4 is precisely feedback-controlled so that the upstream pressure (P1) detected by the first pressure detector 5a becomes a predetermined pressure. As a result, the flow rate (Q) passing through the restriction part OR can be precisely controlled to the predetermined flow rate. A piezoelectrically actuated control valve, a solenoid valve, or the like, which is capable of precise control, is used as the control valve 4.
Under a non-critical expansion condition, the following relation holds true: flow rate Qc=K2P2m(P1−P2)n (K2 is a proportionality coefficient depending on the kind of fluid and the fluid temperature, and exponents m and n are values derived from the actual flow). The downstream pressure (P2) is detected by a second pressure detector (not shown) separately provided on the downstream side of the restriction part OR. Under a non-critical expansion condition, using the above relational expression that holds true under a non-critical expansion condition, the flow rate can be determined by computation from the output of the first pressure detector 5a on the restriction part OR upstream side and the output of the second pressure detector on the restriction part OR downstream side, and the flow controller controls the degree of opening/closing of the control valve so that the determined flow rate becomes the same as the set flow rate (Patent Document 2, etc.).
However, due to influences of a pressure regulating valve (not shown) and the like disposed upstream from the flow controller of this type, the pressure of the gas G flowing through the flow passage 2 may periodically oscillate, resulting in the hunting (pulsation) of the pressure detected by the first pressure detector 5a. The hunting of the detected pressure makes the flow control unstable.
Conventionally, in order to suppress such hunting, software-wise measures and mechanical measures are known. As software-wise measures, for example, the coefficient used in computation by the controller of a flow controller is changed and optimally controlled to suppress hunting (e.g., Patent Document 3, etc.). However, with such software-wise measures, it is not easy to derive the optimum value of the coefficient to be changed.
As mechanical measures, for example, a technique in which a distribution plate having a flow straightening effect, such as a metal mesh plate or an orifice plate, is interposed in a flow passage to distribute a gas flow, or a technique in which a chamber that expands the cross-sectional area of a flow passage is formed in the middle of a flow passage, thereby absorbing pressure fluctuation, are known (e.g., Patent Documents 4 to 6, etc.).